Model-reference adaptive steering control of a farm tractor with varying hitch forces

This paper presents a model-reference adaptive controller (MRAC) for steering a farm tractor with varying hitch forces. Hitch forces play an important role in the yaw dynamics of the tractor, and these forces change with respect to different implements and soil conditions. It is desired that the tractor has the same dynamic response no matter what the hitch forces may be. A good dynamic yaw model has been developed for a tractor with hitch forces, and one parameter in this model changes with hitch loading. A MRAC algorithm is consequently proposed to directly adapt one controller parameter to changing hitch loading. A set of cascaded controllers will be used to regulate the tractor´s lateral position, although only the controller that directly regulates the yaw rate will be adapted. The adaptation algorithm is then augmented to account for inner-loop steering actuator dynamics and saturations. Simulated and experimental results are presented.